There are now commercially marketed in the United States heat sink-mountable rectangular film-type power resistors having molded environmentally-protective synthetic resin housings. These resistors have flat lower surfaces that are at least partially formed by portions of such housings. The lower surfaces have predetermined sizes and shapes that are called "footprints" in the industry. These footprints are defined by power package styles such as TO-220 style and TO-126 style as well as others. These power package styles have been used for years for power semiconductor devices and in recent years for power resistor devices. The thermal mounting issues related to transferring heat from these devices, with these footprints, to a metal heat sink or metal chassis are well understood.
It is desired by users of the resistors that the sizes and shapes of the footprints remain the same. This does not mean that there is only one size and shape of rectangular footprint in the industry; it does mean that there are a limited number of footprint sizes and shapes that are in effect standard in the industry.
Reference is made to the 1987 book named D.A.T.A. Power Semiconductors, Edition 21, published by D.A.T.A. Incorporated. This book summarizes the JEDEC registrations (the assignment of a number to each particular package with its associated footprint). Particular reference is made to page 1,021 of such book relative to the above-indicated number TO 220, and to page 1,012 relative to the above-indicated number TO 126. As there specified, the TO 220 has a maximum footprint width of 0.4197 inch, and a maximum footprint length of 0.65 inch. The TO 126 has a maximum footprint width of 0.330 inch, and a maximum footprint length of 0.450 inch. Also as there specified, somewhat smaller footprint widths and lengths are permitted relative to each number. The above-specified maximum footprint sizes are standard in the semiconductor industry in the United States.